1. Field of the Invention
A method of hydrolyzing fats and oils to produce fatty acids having a low proportion of trans-isomer fatty acids. Specifically, the present invention relates to a process for hydrolyzing fats and oils under conditions resulting in a low proportion of trans-isomer fatty acids.
2. Description of the Related Art
The term “fatty acids” is commonly understood to refer to the carboxylic acids naturally found in animal fats, vegetable, and marine oils. They consist of long, straight hydrocarbon chains, often having 12–22 carbon atoms, with a carboxylic acid group at one end. Most natural fatty acids have even numbers of carbon atoms. Fatty acids may or may not contain carbon-carbon double bonds. Those without double bonds are known as saturated fatty acids, while those with at least one double bond are known as unsaturated fatty acids. The most common saturated fatty acids are palmitic acid (16 carbons) and stearic acid (18 carbons). Oleic and linoleic acid (both 18 carbons) are the most common unsaturated fatty acids.
Trans fatty acids are unsaturated fatty acids that contain at least one double bond in the trans isomeric configuration. The trans double bond configuration results in a greater bond angle than the cis configuration. This results in a more extended fatty acid carbon chain more similar to that of saturated fatty acids rather than that of cis unsaturated double bond containing fatty acids. The conformation of the double bond(s) impacts on the physical properties of the fatty acid. Those fatty acids containing a trans double bond have the potential for closer packing or aligning of acyl chains, resulting in decreased mobility; hence fluidity is reduced when compared to fatty acids containing a cis double bond. Trans fatty acids are commonly produced by the partial hydrogenation of vegetable oils
It has long been known that high dietary levels of saturated fatty acids are linked to increased total and low-density lipoprotein (LDL) cholesterol concentrations. More recently, however, a number of studies have reported that a diet rich in trans-isomer fatty acids not only increased LDL concentrations but also decreased high-density lipoprotein (HDL) cholesterol concentration, resulting in a less favorable overall total cholesterol/HDL cholesterol ratio (Aro et al, Am. J. Clin. Nutr., 65:1419–1426 (1997); Judd et al, Am. J. Clin. Nutr., 59:861–868 (1994); Judd et al, Am. J. Clin. Nutr., 68:768–777 (1998); Louheranta et al, Metabolism 48:870–875 (1999); Mensik and Katan, N. Engl. J. Med. 323:439–445 (1990); Muller et al, Br. J. Nutr. 80:243–251 (1998); Sundram et al, J. Nutr. 127:514S–520S (1997)). Recent data has further demonstrated a dose-dependent relationship between trans-isomer fatty acid intake and the LDL:HDL ratio and the magnitude of this effect is actually greater for trans-isomer fatty acids compared to saturated fatty acids (Ascherio et al, N. Engl. J Med. 340:1994–1998 (1999)).
Naturally occurring fats and oils contain triesters of glycerol and three fatty acids. Hence, they are referred to chemically as triacylglycerols or, more commonly, triglycerides. The fat or oil from a given natural source is a complex mixture of many different triacylglycerols. Vegetable oils consist almost entirely of unsaturated fatty acids, while animal fats contain a much larger percentage of saturated fatty acids. Fats and oils are used in a wide variety of products, such as soaps and surfactants, lubricants, and in a variety of other food, agricultural, industrial, and other personal care products.
Triacylglycerols, like all esters, can by hydrolyzed to yield their carboxylic acids and alcohols. The reaction products produced by the hydrolysis of a fat or oil molecule are one molecule of glycerol and three molecules of fatty acids. This reaction proceeds via stepwise hydrolysis of the acyl groups on the glyceride, so that at any given time, the reaction mixture contains not only triglyceride, water, glycerol, and fatty acid, but also diglycerides and monoglycerides.
Currently, the most commonly used commercial process for hydrolyzing fats and oils is a high-temperature steam treatment method known as the Colgate-Emery Steam Hydrolysis Process (Brady, C., L. Metcalfek, D. Slaboszewski, and D. Frank, JAOCS, 65:917–921 (1988)). This method, and modifications thereof, use a countercurrent reaction of water and fat under high temperatures ranging from 240° C. to 315° C. and high pressures in the range of 700 to 750 PSIG. Presently, the Colgate-Emery process is the most efficient and inexpensive method for large-scale production of saturated fatty acids from fats and oils. In this method, a tower is used to mix the fat and water to increase the efficiency of the hydrolysis reaction. The fat is introduced from the bottom of a tower with a high pressure feed pump. Water is introduced from near the top of the tower at a ratio of 40–50% of the weight of the fat. As the fat rises though the descending water, a continuous oil-water interface is created. It is at this interface that the hydrolysis reaction occurs. Direct injection of high pressure steam raises the temperature to approximately 260° C. and the pressure is maintained at 700–715 PSIG. The increased pressure causes the boiling point of the water to increase, allowing for the use of higher temperatures, which results in the increase of the solubility of the water in the fat. The increased solubility of water provides for a more efficient hydrolysis reaction. This continuous, countercurrent, high pressure process allows for a split yield of 98–99% efficiency in 2–3 hours (Sonntag, JAOCS 56: 729A–732A (1979)). Further purification of the fatty acid product obtained by this method is often accomplished by means such as distillation.
However, due to the extreme reaction conditions, this process often leads to extensive degradation of the produced fatty acids. For example, the Colgate-Emery method has not been shown to be effective in splitting heat sensitive triglycerides containing conjugated double bonds, hydroxy-containing fats and oils like castor oil, fish oils containing polyunsaturated acids and soybean oils high in unsaturated fats due the formation of by-products such as trans-isomer fatty acids and the degradation of the unsaturated fatty acids at high temperatures (Sonntag, JAOCS 56: 729A–732A (1979)). Therefore, the production of fatty acids from vegetable oils (e.g., soya, corn and peanut), which are generally high in unsaturated fats, is not recommended by this method.
Some sectors of industry have used other methods of hydrolysis to avoid the by-product formation and unsaturated fat degradation associated with the high pressure-high temperature hydrolysis of unsaturated fats and oils. These include the hydrolysis of unsaturated oils by splitting them with a base followed by acidulation or by enzymatic hydrolysis. However, none of these methods have shown split yields comparable to the Colgate-Emery process under similar time conditions.
In light of the limitations of the current methods used for the hydrolysis of unsaturated fats and oils, a need in the art exists for an efficient method of non-catalytic hydrolysis suitable for unsaturated fats and oils which produces fatty acid products with a low percentage of trans-isomer fatty acids.
Moreover, the U.S. Food and Drug Administration has initiated the process of requiring food labels to include the trans-isomer fatty acid contents of food. As such, there is an incentive for food manufacturers to decrease the trans-isomer fatty acid content of their products. Thus, a need has developed for methods of hydrolysis of unsaturated fats and oils that provide for the production of fatty acids with a low proportion of trans-isomer fatty acids for use as food products.
The present invention addresses these needs by providing a method of hydrolyzing fats and oils high in unsaturated fat whereby the fatty acid products have a low trans-isomer fatty acid content suitable for use in the food industry.